The Dreamkey is a GD-ROM that was supplied with Sega's now discontinued Dreamcast games console. Using this disc, owners of the Dreamcast were able to connect to the Internet, being able to send or receive email, surf the web, and - most importantly - play supported games against one another. Two versions were released bundled with the console: one being just the Dreamkey GD, the other containing the first volume of the Dreamon series of demonstration discs.
The software has gone through a number of revisions, with version 3.0 being the final version released. The version bundled with the console, and the most common, is version 1.0.
Using the Dreamkey bundled with your DC is straightforward. Place the GD into the console and power it up, and after the familiar boot screens, a screen will appear displaying the Dreamkey logo, the logos of Sega and the supporting companies (such as BT), and a flashing "press start" icon. Doing so brings you to the language selection screen; as well as deciding what language to present the following screens in, this also chooses the phone number that will be dialed.
A number of options can be chosen from the next screen, such as tone or pulse dialing. There is also the "AT options" option, which you are told you really shouldn't be using. Best to ignore the whole option, really. Once all this has been set, continuing will commence dialing.
During the DC's hayday, this would have happily dialed the necessary number and connected to the 'net, allowing you to register with Dreamarena, the online gaming service. However, this is no longer possible, due to Sega having dropped Dreamcast support.
However, if you are desperate to get online using a Dreamcast, there is hope. Calling Sega technical support (in the UK: 08456 090 090) and asking for Dreamkey version 3 will result in a new browser disc being sent to the address you give them. The advantage of Dreamkey 3.0 is that it allows you to enter in any ISP details you like - telephone number, username and password - and connect via your Dreamcast.